San Francisco International Airport officials have taken proposed expansion plans "back to the drawing board," in the words of one official, because the planned development would threaten the habitat of an endangered species of snake. A three-year study by the state Department of Fish and Game and the U.S. Wildlife Service recommended that 120 acres across the Bayshore Freeway from the airport in San Bruno should not be developed because doing so would require moving the snakes. Airport Director Lou Turpen said the San Francisco garter snake should not be transplanted from its natural habitat on the airport property for fear it might become extinct. The snake, he said, is unique to this strip of land, characterized by marshes, creeks and canals. Researchers, he said, have found and tagged 635 of the snakes so far.